Using cached connector directory for discoverable tools list (#21497)

## Summary

Startup tool construction currently depends on connector directory
metadata for `tool_suggest` discoverables. On a cold directory cache,
that can put slow connector-directory requests on the blocking path even
though the tools array only needs directory data for install
suggestions, not for the live connector MCP tools themselves.

This PR keeps the discoverables path off that cold network fetch:
- read connector directory metadata from cache only when building
discoverable tools
- persist connector directory metadata to
`~/.codex/cache/codex_app_directory/<hash>.json` and use it to hydrate
the in-memory cache on later runs before the normal refresh path updates
it
- use connector-directory-specific cache naming to distinguish this
metadata cache from the separate Codex Apps tools-spec cache

This reduces first-turn startup work without changing how live connector
MCP tools are sourced. Longer term, directory-backed install suggestions
should move to a search-based flow so they no longer need to be inlined
into the tools prompt at all.

## Testing

- `cargo test -p codex-connectors`
- `cargo test -p codex-chatgpt`
- `cargo test -p codex-core
request_plugin_install_is_available_without_search_tool_after_discovery_attempts`
- `cargo test -p codex-core
tool_suggest_uses_connector_id_fallback_when_directory_cache_is_empty`

codex-rs/core/src/connectors_tests.rs

@@ -19,6 +19,7 @@ use codex_connectors::metadata::connector_install_url;
 use codex_connectors::metadata::connector_mention_slug;
 use codex_connectors::metadata::sanitize_name;
 use codex_features::Feature;
+use codex_login::CodexAuth;
 use codex_mcp::CODEX_APPS_MCP_SERVER_NAME;
 use codex_mcp::ToolInfo;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -1120,6 +1121,42 @@ disabled_tools = [
     );
 }
 
+#[tokio::test]
+async fn tool_suggest_uses_connector_id_fallback_when_directory_cache_is_empty() {
+    let codex_home = tempdir().expect("tempdir should succeed");
+    std::fs::write(
+        codex_home.path().join(CONFIG_TOML_FILE),
+        r#"
+[features]
+apps = true
+
+[tool_suggest]
+discoverables = [
+  { type = "connector", id = "connector_gmail" }
+]
+"#,
+    )
+    .expect("write config");
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .build()
+        .await
+        .expect("config should load");
+    let auth = CodexAuth::create_dummy_chatgpt_auth_for_testing();
+
+    let discoverable_tools =
+        list_tool_suggest_discoverable_tools_with_auth(&config, Some(&auth), &[])
+            .await
+            .expect("discoverable tools should load");
+
+    assert_eq!(
+        discoverable_tools,
+        vec![DiscoverableTool::from(plugin_connector_to_app_info(
+            "connector_gmail".to_string(),
+        ))]
+    );
+}
+
 #[test]
 fn filter_tool_suggest_discoverable_connectors_keeps_only_plugin_backed_uninstalled_apps() {
     let filtered = filter_tool_suggest_discoverable_connectors(